If you're working a lot with extensions you often stumble upon problems, when you want to include them into your custom web application, like hardcoded or absolute path aliases or classes extended from core application components, which implement addiditonal features.

When using themes you want to changes some of the translations specifically to the theme while keeping the standard translations for most of the text.
Duplicating the main translation file is a headache for maintainance.
This how to provides a solution.